Liquid crystal display devices comprising polarizers and micro-color filter components are utilized in various flat panel displays. Reflective displays with full color capability are currently top-of-the-line products for portable electronics. Such reflective full color performance meets its basic requirement of high-information-content displays for a simple reason of less power consumption and thinner structure compared with the backlit counterparts. The typical reflective displays, nowadays, are reflective thin film transistor (TFT) display and reflective STN display. However, the overall performances of the reflective displays are still not as good as the transmissive backlit mode in terms of brightness, contrast ratio and viewing angle. And it is difficult to achieve the same contrast practically available for a transmission backlit display. These above-mentioned disadvantages of reflective TFT and STN displays result mainly from a light-loss by the absorptive polarizer and from the angular dependency of the axis of polarization. In general, there is more than 60% optical loss. In the case of color display, light-loss is further aggravated due to the absorptive color filter which will cut off at least 60% incoming light.
U.S. Pat. No. 4,032,218 introduces a cholesteric color reflector and TN cell to display monochrome information on the black background. A quarter-wave plate is positioned between the cholesteric film and the TN display to convert circular polarization into linear polarization. A black coating is attached on the back of the device to absorb all the residual light passing from the cholesteric film. As a result, a viewer will sense a bright color light generated by the cholesteric color film on a black background.
U.S. Pat. No. 5,555,114 teaches cholesteric color selection layer, which selectively reflecting circularly polarized light of a specific wavelength and an optical layer formed on the color selection layer and having a liquid crystal and means for applying an electric field to the liquid crystal layer. A linear optical shifting layer on the top of cholesteric color filter convert circularly polarized light into linear polarization. This approach is not sufficient for a STN cell, a non-wave-guiding mode display, because of its non-linear optical performance due to the super twist dispersion to the incoming light. The color is actually the combination of color dispersion of birefringence of display cell and Bragg reflection from cholesteric color selection layer. In order to eliminate the color dispersion, different voltage will apply to different color pixels to convert the elliptical polarization into circular polarization, yet this make driving scheme very complicate or even impracticable.
U.S. Pat. No. 5,949,513 teaches a method of manufacturing a multi-color cholesteric display. The method includes the steps of: (1) deposition a twist agent on a first substrate, the twist agent becoming an in situ twist agent, (2) bringing a second substrate into proximity with the first substrate to form at least one interstitial region between the second and first substrates, (3) introducing liquid crystal having an initial pitch into the at least one interstitial region proximate the in situ twist agent and (4) stimulating the LC and the in situ twist agent to cause the LC and the in situ twist agent to mix in situ, the in situ twist agent to mix in situ, the in situ twist agent changing the initial pitch of the LC. A permanent polymer wall is necessary to isolate the LC of different pitch from flowing around. The multi-color cholesteric display takes advantage of the cholesteric Bragg reflection with spatial variable wavebands. But the shortcoming is the limited brightness due to the fact that only one circular polarization has been used.
U.S. Pat. No. 6,285,434 teaches a method of manufacturing a multi-color cholesteric display. The method introduces a substrate having a cell wall structure that enhances manufacturability by isolating the fluid fill ports corresponding to each set of independent cells, whereby each set of independent cells can be selectively-filled with a liquid crystal having desired properties. Thus, three colors of cholesteric liquid crystal material will be filled into the isolated channels sequentially by vacuum and/or capillary approaches. Again, the resulting cholesteric display is not bright enough compared with the color newspaper in the ordinary ambient environment due to the fact that only one handedness of the incident light has been utilized.
The applicant and the other research groups have tried multi-layers of cholesteric display cell structure in order to gain higher brightness for the full color display. But it causes very high complication and cost in terms of driving electronics and manufacturing process. The multi-layer itself adds more surface or interfacial reflection and absorption. As a result, the performance of such a multiple-layer-approach is, so far, not really satisfactory.